In refining of metallurgical grade silicon for production of solar grade silicon it is known to remove boron from molten silicon by treatment of molten silicon with calcium-silicate based slag. In order to remove boron from molten silicon to obtain an acceptable low content of boron in the silicon it is necessary to use a slag having a being very low boron content.
Slag treatment of silicon for boron removal also has effect on the phosphorus content in the silicon as the distribution coefficient between phosphorus in the slag and in molten silicon is very low and in the range of 0.1 to 0.3. If calcium-silicate based slag used for removal of boron from molten silicon has a high phosphorus content, the phosphorus content in the silicon may thus increase during the slag treatment process. The phosphorus content in silicon grade silicon should be lower than 3 ppmw and it is therefore important to use a calcium-silicate based slag with a lowest possible phosphorus content during the slag treatment for removal of boron.
When producing calcium-silicate based slag from a source of calcium and SiO2, it is difficult to find a source of calcium having a sufficiently low content of phosphorus in order to produce calcium compounds with a sufficiently low phosphorus content. In addition most sources of calcium have a low, but notifyable boron content.
From WO 03/097528 it is known a method for removing of phosphorus from calcium-silicate based slag where low price sources of CaCO3 and SiO2 can be used to produce the slag. By the method of WO 03/097528 calcium-silicate based slag is treated with a molten ferrosilicon alloy whereby phosphorus in the calcium-silicate based slag is transferred to the ferrosilicon alloy whereafter calcium-silicate based slag with a low phosphorus content is removed from the molten ferrosilicon alloy.
By the method of WO 03/097528 it is, however, not possible to remove boron from the slag. The boron content of the calcium-silicate based slag is thus directly dependent on the boron content of the raw materials used to produce the calcium-silicate based slag. In addition the method disclosed in WO 03/097528 is costly as it is performed at high temperatures, resulting in high energy consumption.
It is thus a need to produce calcium compounds for use as a raw material for production of calcium-silicate based slag having both a very low phosphorus content and a very low boron content.
By the present invention it has now been found a simple method whereby phosphorus can be substantially removed from a source a calcium and where also the boron content can be reduced.